1. Field of the Invention
This invention relates to the preparation of dye solutions where the dye is a Lewis acid, for example, the metal chelate dyes or acid dyes containing acid groups such as carboxy, sulfo or hydroxy. These dye solutions may be used with a variety of solvents in which the dye was previously only slightly soluble. More importantly, it is now possible to prepare dye solutions in solvents which are compatible with reactive urethane prepolymers.
2. Description of the Prior Art
The increasing popularity of polyurethane in the coating industry can be attributed to the wide latitude of tactile and visual effects which careful chemical design has made possible as well as to the recognized superior physical performance which urethanes have.
It is well known in the art that in the formulation of metal complex dye solutions for use with conventional lacquers, dyes can be dissolved in alcohols and alcohol-like solvents such as the lower alkanols including propanol and the like or in methyl cellosolve. These solutions, however, cannot be employed with reactive isocyanate systems because of the presence of alcohol solvents which are reactive with isocyanate groups.
It is well known to employ primary, secondary, or tertiary amines to increase the solubility of the dyes in the solvents employed with the conventional lacquers. However, primary and secondary amines react with isocyanates and the tertiary amines, which can not combine with the isocyanates, cause trimerization and polymerization, especially where the isocyanate is an aromatic isocyanate such as toluene diisocyanate, methylenebis (phenylisocyanate) or an isocyanate prepolymer prepared therefrom. Therefore, the ordinary amines employed with the conventional lacquers cannot be employed with the reactive polyurethanes.
Reactive polyurethanes are defined as those polyurethanes which contain unreacted isocyanate groups. Generally, the isocyanates are present as prepolymers although monomeric isocyanate containing materials may be employed. Not high enough in molecular weight to be classified as a polymer, a prepolymer is described as a partially reacted isocyanate which still contains unreacted isocyanate groups that can combine with moisture or other curing agents such as multi functional alcohols, amines or acids to produce a true polyurethane. A complete description of the prepolymers is found in U.S. patent application Ser. No. 7,270 filed Jan. 30, 1970, now U.S. Pat. No. 3,743,626, application is hereby incorporated by reference.
To dissolve urethanes, it is well known to employ solvents which do not contain groups which can react with isocyanates such as esters, ethers, ketones or aliphatic and aromatic hydrocarbons. However, until now, these solvents could not be employed with the metal chelate dyes due to the low solubility of these dyes in said solvents.
The superiority of the metal chelate dyes is well recognized in the art because of their high color strength, brilliance and transparency. Also, said dyes have exceptional fastness to light, heat and chemicals. Therefore, there has been a continuing search for a solvent system to permit the use of the metal chelate dyes with reactive polyurethane systems. Further, even is systems where it is not necessary to exclude the polyurethane reactive solvent, it may be desirable to be able to work with more concentrated solutions of dyes in other solvents. Thus, a material which substantially increases the solubility of a dye is desirable.